Here's the link to the original article. I found this buried in my email from forever ago, thought I'd share it:
http://news.discovery.com/history/gladiator-tombstone-epitaph-110621.html
So apparently Diodorus with the two swords downed Demetrius, but the ref didn't declare it a submission, so Demetrius was allowed to get back up, at which point he killed Diodorus. Gotta make those calls right, refs!
